The overall objective is to characterize the DMBA-induced hyperplastic lesions of the rat mammary gland and to examine their preneoplastic potential. Comparison will be made of the several morphological types induced by the carcinogen, and to a lesser extent, hyperplasias occurring spontaneously in old rats will also be examined. One major approach is to establish several transplantable outgrowth lines of hyperplastic lesions in the mammary fat pads of isologous hosts, which will allow a study of their stability, morphology, growth patterns, tumor producing potential, and hormone-dependence and responsiveness. The second major approach will involve the use of the whole gland organ culture system to study the induction in vitro of mammary hyperplasias by DMBA. This approach will facilitate analysis of the interactions of carcinogen, hormones, and isolated mammary target tissue, which should help to elucidate mechanisms and control of cell transformation.